Multi-faceted scanners usually comprising multi-faceted rotating mirrors are employed in well known techniques for erecting optical scanning between a light source and a photocell. Typically, a light illuminates a silvered mirror, for example, at an angle of 45.degree. to direct light toward a facet that is reflected from the facet toward the object being scanned. Normally the object reflects this light back along the same path upon a photocell. The duration of the scan corresponds to the time for a facet to pass the light beam along the object being scanned. It is usually preferred that the object path scanned is independent of which facet is then in the light beam path.
In connection with television equipment, it is known to use mirror prisms for image scanning along one dimension, usually for line scanning. Since the advent of television, cameras operating in accordance with the image storage system, the need for such mirror prisms has become greatly increased. Recently television cameras have been designed for operation within the infrared radiation range, for example, within the range of 2 to 5.5 microns. Television cameras operating within this wave-length require mirrors or similar light deflecting optical means for scanning an image. Usually one means, for instance, a light deflecting mirror, is used for vertical scanning image division. Rotary mirror prisms which are generally prisms composed of several plane mirrors such as glass mirrors are conventionally employed by suitably mounting them on a shaft or other rotary support. These mechanically composed rotary prisms are found to have many disadvantages, both as to their optical characteristics and their mechanical reliability. In particular, they have been found mechanically difficult to mount the several planed mirrors so that they accurately form a polygonal shape of predetermined dimensions. For short optical path lengths, slight misalignment of the facets is found to be of little practical significance. However, when the distance between the scanning mirror and the object being scanned is many feet, slight misalignment of the facets results in the path of scan changing from one facet to the other. Such a result is especially disadvantageous when scanning labels with an encoded stripe arrangement. If there is misalignment of the facets one facet might make a perfect scan of the coded stripes while the next facet would register no scan at all or only scan a few of the stripes.
Morever, it is difficult to mount the mirrors so that they accurately retain their spatial positions when subjected to the stresses of high speed rotation. The last mentioned mounting problem entails a danger of injury to persons close to the spinning mirror prism which is often unavoidable. Obviously when the mirror prism should disintegrate shrapnel is produced which may cause serious injury to a bystander.
Thus, many methods have been investigated to produce multi-faceted scanners so that the materials from which they are composed would have high modulus to density ratio, low thermal expansion, low Poisson's ratio, good workability and possess the ability to be readily polishable or coatable with a substance which in turn can be polished to produce high quality optical surfaces. Unfortunately, the imposition of these material restrictions result in the requirement of a material which is not readily available. Presently, in view of these material restrictions and limitations, scanners are now being manufactured from glass, stainless steel, beryllium and chromium carbide. The latter two materials are the most widely used since they more nearly meet the requirements of the predicated material limitations. Of these two, beryllium is found to best satisfy the material requirements of the predicated material limitations and consequently is found to perform in a superior fashion when employed. However, the use of beryllium to provide multi-faceted scanners in and of itself results in still other problems among which are exorbitant cost of the material and the extreme difficulty of working the material into the desired configurations. Chromium carbide scanners, although not as expensive as beryllium scanners, possess very high density and therefore require in the overall general construction of the scanner a driver motor and bearings which are much heavier and much more costly to provide.
There is therefore a demonstrated need to provide multi-faceted scanner systems which may be precisely machined, inexpensively, and with great facility than known scanner systems enabling these multi-faceted scanners to be considered for employment in a vast number of applications other than military or development laboratories where the exorbitant costs of currently available scanner systems can only be justified.
It is therefore an object of this invention to provide a novel multi-faceted scanning system devoid of the above noted deficiencies.
It is another object of this invention to provide a novel multi-faceted scanner capable of operation at high rotational speeds.
It is another object of this invention to provide a novel multi-faceted scanner system characterized by precise alignment of the facets.
Another object of this invention is to provide a novel scanning system which achieves precise alignment of the different facets with techniques that are relatively easy to perform.
These and other objects of the system of the instant invention are accomplished, generally speaking, by providing acrylic high speed multi-faceted scanners by injection molding. Injection molded acrylic has been used for the production of lowcost, medium quality lenses. However, it has not been practical to employ acrylics in reflecting optics due to its low adhesion to thin film coatings such as aluminum. Since the advent of the application of magnesium fluoride as an overcoating to acrylic substrates, it has now been made possible to properly adhere surfaces which possess the proper reflecting optics or reflectivity to the acrylic substrates employing techniques more fully described in copending application Ser. No. 687,962, filed May 19, 1976.
Two methods, for example, that may be employed in providing injection molded high speed multifaceted scanners include: providing an aluminum hub which is placed into a die cavity having the proper facet geometry. The aluminum hub is sized so as to provide a suitably dimensioned injection ring having suitable mechanical properties and optical properties for high speed scanning applications. Acrylic is injection molded into the gap between the aluminum hub and the die cavity. In order that the resulting member represent a stressfree ring at room temperature, the aluminum disks have to be preheated to approximately 500.degree. F, for example, or roughly 100.degree. F above molding temperature to accommodate the slightly high acrylic shrink rate. The effect of the preheat will be minimal on the physical properties of the aluminum alloy employed of which aluminum 7075-T651 is preferred. The configuration of the scanner showing the adaption of molded acrylic to the aluminum disk workpiece is seen in FIG. 1 which will be specifically discussed later. A variation of this first method which is less complicated and expensive eliminates the aluminum resulting in a solid acrylic injection molded scanner.
In another embodiment the acrylic ring is molded separately and then cemented onto an aluminum hub fabricated of the aluminum alloy recited above. In FIG. 2 which is later discussed, the specifics of this method and configuration are more specifically outlined.
In both cases the acrylic is coated with magnesium fluoride and then a mirror-like finish of aluminum is applied with a protective coating of, usually, silicon monoxide to complete the process if desired. The 7075 aluminum disk above is found to be rotational at actual speeds of about 123,000 rpm although higher rates are obtainable. The strength and other characteristics of the acrylic disk is quite different from the aluminum disk. The maximum rpm that the aluminum acrylic polygon can be rotated is somewhat lower than about 40,000 rpm for scanners produced by the first method and somewhat higher than about 40,000 rpm for scanners produced by the second method. This difference is due to the fact that in the first method a fraction of the strength of the acrylic is used to hold the ring on the disk while in the second method the strength of the ring is enhanced by the cement used to secure the acrylic in place. The above values have been derived from equation which follows: ##EQU1## where
S.sub.(t) = maximum tangential stress (PSI) at the bore
.gamma. = weight per inch.sup.3 (0.05)
G = gravitational constant
.omega. = radians per second
It should be noted however that for medium and low speed scanning applications the scanner may be fabricated of injected molded acrylic eliminating the need for an aluminum alloy preform.
7075-T651 aluminum alloy is recommended when extra strength and hardness are required. It is used primarily for aircraft and ordinance applications. The preferred aluminum alloy for use in this system is found to have the following properties:
______________________________________ Nominal Chemical Composition ______________________________________ Zinc 5.6% Magnesium 2.5% Copper 1.6% Chromium 0.3% Aluminum Balance (incl. normal impurities) Typical Tensile Strength, psi 83,000 Mechanical Yield Strength, psi 73,000 Properties Elongation, % in 2" 11 Shear Strength, psi 48,000 Brinell Hardness 10/500 150 Typical Density, Lbs./Cu. In. 0.101 Physical Melting Range, approx. .degree. F 890-1180 Properties Electrical Conductivity, % IACS at 20.degree. C (68.degree. F) 33 Thermal Conductivity, btu at 25.degree. C (77.degree. F) 900 Average Coefficient of Thermal Expansion at 68.degree. to 212.degree. F 0.0000131 ______________________________________
These typical properties are average values.
______________________________________ Fabricating Performance Cold Forming: Poor Machining: Good Brazing: Not suitable Welding: Arc, Poor Gas, Poor Resistance, Good Government & Industry Specifications Cold Finish-Rolled Extruded ______________________________________ A.M.S. 4122C, 4123A 4154F, 4168A, 4169B A.S.T.M. B211 B221 Federal QQ-A-225-9b(QQ-A-282) QQ-A-200/11b(QQ-A-277) Military None None S.A.E. AA7075 AA7075 ______________________________________